Theory for One Type of Superconductor Solves Puzzle in Another

A 2017 theory proposed by Rice University physicists to explain the contradictory behavior of an iron-based high-temperature superconductor is helping solve a puzzle in a different type of unconventional superconductor, the “heavy fermion” compound known as CeCu2Si2.

An international team from the U.S., China, Germany and Canada reported the findings this week in the Proceedings of the National Academy of Sciences (PNAS). The study focused on a cerium, copper and silicon composite whose strange behavior in 1979 helped usher in the multidisciplinary field of quantum materials.

That year, a team led by Max Planck Institute’s Frank Steglich, a co-author on the PNAS paper, found that CeCu2Si2 became a superconductor at extremely cold temperatures. The mechanism of superconductivity couldn’t be explained by existing theory, and the finding was so unexpected and unusual that many physicists initially refused to accept it. The 1986 discovery of superconductivity at even higher temperatures in copper ceramics crystalized interest in the field and came to dominate the career of theoretical physicists like Rice’s Qimiao Si, a PNAS study co-author and the Harry C. and Olga K. Wiess Professor of Physics and Astronomy.

Si, whose decadeslong collaboration with Steglich has led to almost two dozen peer-reviewed studies, said, “In my wildest dreams, I had not thought that the theory that we proposed for the iron-based superconductors would come back to the other part of my life, which is the heavy-fermion superconductors.”

Heavy fermions, like high-temperature superconductors, are what physicists call quantum materials because of the key role that quantum forces play in their behavior. In high-temperature superconductors, for example, electrons form pairs and flow without resistance at temperatures considerably warmer than those needed for conventional superconductivity. In heavy fermions, electrons appear to be thousands of times more massive than they should.

In 2001, Si, who also directs the Rice Center for Quantum Materials (RCQM), offered a pioneering theory that these phenomena arise at critical transition points, tipping points where changes in pressure or other conditions bring about a transition from one quantum state to another. At the tipping point, or “quantum critical point,” electrons can develop a kind of split personality as they attempt to straddle the line between states.

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